Acoustic imaging systems incorporate acoustic transducers for converting electrical signals into mechanical pressure or particle displacement signals and vice versa. The conversion is done typically by a piezoelectric ceramic, or in the case of transducer arrays by an array of ceramics. The plane defined by the axis of the array and the normal to the array's active surface is known as the azimuthal plane and the plane orthogonal to the azimuthal plane is known as the elevation plane. In the azimuthal plane, steering, focusing and aperture control are accomplished electronically by the imaging system through applying appropriate delay, phase and apodization to the individual array elements. An example of an acoustic imaging system can be found in U.S. Pat. No. 4,550,607 (Maslak et al.), for example.
For one-dimensional arrays, elevation plane focusing can generally be categorized as either lens focused or mechanically focused. In the case of lens focused arrays, the active emitting surface of the array is flat in the elevation plane and a shaped lens is placed between the object to be imaged and the active surface of the array. U.S. Pat. Nos. 4,686,408 and 5,163,436 describe lens focused phased array transducers. The material used to form the lens is typically silicone based and, unfortunately, also has the undesirable property of absorbing or attenuating passing ultrasound energy and thereby reducing the overall sensitivity of the transducer array. Mechanically focused transducer arrays involve curving the active surface of the transducer array along the elevation direction. The elevation aperture size and elevation focus depth for lens and mechanically focused transducer arrays, however, remains fixed.
For one-and-a-half dimensional arrays (1.5-D array) and two-dimensional arrays (2-D array), on the other hand, steering angle and focus depth in elevation, typically to a limited extent, and elevation aperture size are also controllable electronically by the imaging system. 1.5 and 2-D arrays require, respectively, 2 to 4 times and 16 to 64 times more number of acquisition channels compared to one dimensional arrays. Therefore a much more complex and expensive system hardware is required. They, however, offer better control over elevation beam width (slice thickness) which potentially improves detectability of targets that have a small extent in elevation. 2-D arrays also allow three-dimensional imaging.
Some of the basic design parameters considered when designing a transducer array include the center frequency, bandwidth, elevation aperture size, elevation focal depth and element spacing. Center frequency and bandwidth define the pass-band of the transducer impulse response. The frequency of operation, together with the aperture size, determine the lateral resolution of the beam both in azimuth and elevation, and the beam's penetration. Therefore, for imaging shallow structures where penetration is not an issue, the operating frequency should be high to maximize detail resolution. However, to image deep, the operating frequency has to be low in order to penetrate. The absolute bandwidth for any given operating frequency determines the axial resolution at focus. For a given operating frequency, elevation aperture size and elevation focal depth determine the focusing in elevation. For high frequency operations which are limited to imaging shallow structures, the elevation aperture should be small and focusing depth should be shallow to maximize contrast resolution in the near field. For low frequency operations, however, elevation aperture should be large and focusing depth should be deep for the best resolution and signal-to-noise ratio. Element spacing, along with the operating frequency, determines the grating lobe levels and also, given the number of transducer elements, determines the physical aperture size. Therefore, for high operating frequencies where grating lobe levels may be an issue, element spacing has to be small to minimize the grating lobe levels. But for low operating frequencies where grating lobe levels are not an issue, element spacing should be large to maximize the aperture size and thus resolution and penetration.
Barthe, P. G., "Analysis of Tapered Thickness Piezoelectric Ceramics for Ultrasound Transducers," Ph.D. Thesis, Georgia Institute of Technology, 1991, suggests tapering the thickness of a transducer ceramic to achieve very wide bandwidth transducers. But, for such broadband transducers, Barthe does not address the problem of optimizing frequency dependent transducer parameters such as elevation aperture size, elevation focal depth, and element spacing.
U.S. Pat. Nos. 5,415,175 ("the '175 patent") and 5,438,998 ("the '998 patent") describe varying the thickness of a transducer ceramic and matching layers in elevation such that the ceramic is thick at the edges and narrow at the center. With this structure, the elevation aperture becomes a function of frequency and bandwidth; tapered and small at high frequencies and untapered and wide at low frequencies, and the elevation focal depth is determined by the ceramic's thickness profile and the applied bending. This technique works well to achieve a narrow elevation aperture if the frequency is high and the bandwidth is narrow. However, for high frequency/wide bandwidth operations, it is hard to achieve elevation aperture reductions unless a very aggressive edge to center thickness ratio is used. On the other hand, for low frequency/narrow bandwidth operations, and especially if the edge/center thickness ratio is high, the elevation apodization may become inverse-cosine like. This may cause increased elevation side lobe levels for low frequency, narrow bandwidth operations.
U.S. Ser. No. 08/675,412 entitled "Ultrasound Transducer for Multiple Focusing and Method for Manufacture Thereof", filed on Jul. 2, 1996 which is hereby specifically incorporated herein by reference describes varying the thickness of the ceramic and matching layers along the elevation direction such that the elements are thinnest at one end of the array and thickest at the other. This allows for frequency and bandwidth control of the elevation aperture position and size for all operating frequencies and the effective apodization shape is always unimodal. On the other hand the transducer array described the '175 and '998 patents has a fixed aperture position and bandwidth control of the aperture size is only possible at the highest operating frequency and the apodization shape can be bimodal at low operating frequencies when the operation bandwidth is narrow. The '412 application also suggests bending the array along the elevation direction. This allows, if the elements are convex, steering the elevation beam by changing the operating frequency or, if the elements are concave, focusing at a fixed focus at all frequencies. By appropriately designing the ceramic and matching layer thickness as a function of the elevation position, the elevation aperture size can be optimized for each frequency.
It is thus desirable to provide a wide bandwidth transducer that can operate at a wide range of operating frequencies and that optimizes the elevation aperture size, elevation focus depth and element spacing for the frequency of operation. It is also desirable to provide a one-dimensional array that allows electronic control of slice thickness and limited three-dimensional imaging through controlling of frequency and bandwidth.
It is also desirable to provide a two-dimensional transducer that has the same number of transducer elements as a conventional one-dimensional array that can be used to perform three-dimensional imaging without requiring any physical translation of the transducer.